This invention relates to an implantable infusion pump for the dispensing of infusate. In particular, it relates to a pump operating at positive pressure which is programmable to dispense medication in accordance with different specified flow rates.
Implantable infusion pumps are currently used for a variety of medical purposes. Typical of such commercially acceptable devices are the INFUSAID Model 100 and 400 devices. Such devices are implantable in the human body and rely on a liquid/vapor equilibrium to maintain constant pressure on the drug which is housed therein so that the drug flows through a capillary in order to maintain a constant flow rate. Such devices are characterized by "constant flow" and are used in a variety of medical applications, for example, to dispense chemotherapy at a relatively constant flow rate. As background to the INFUSAID Model 100 and 400 devices, are U.S. Pat. Nos. 3,731,681 and 4,496,343.
There are a variety of medical conditions where a patient requires an adjustment in the dosage and as such, constant flow pumps are inadequate. A typical example is diabetes where the quantity of medication, such as insulin, to be infused varies due to requirements of the patient. Fluctuations can occur on a daily basis or more randomly as a function of the ingestion of food. It is known that the amount of medication to be infused per unit of time should be adjusted at certain time intervals. A patients requirements may fluctuate at either set, known rates or vary abnormally, for example, by the ingestion of food or other transitory conditions. Those conditions require the administration of a bolus dose of infusate. The daily administration of insulin requires a basal dose that is supplanted by bolus doses, for example, at meal times. The difference in flow rates between the basal and bolus doses may be quite large, the bolus dose several orders of magnitude larger than the basal dose. Consequently, to achieve proper flow rates over the spectrum of desired rates, such a device must have the ability to continually infuse at very low rates yet provide, periodically a substantially increased flow rate.
Within the reported literature, a number of implantable programmable concepts have been disclosed. Typical are U.S. Pat. Nos. 3,894,538; 4,077,405; and 4,443,218. A category of programmable pumps is so-called negative pressure pumps typified by U.S. Pat. Nos. 4,482,346 and 4,486,190. These devices are solenoid activated negative pressure devices. In those systems a diaphragm storage chamber maintains the drug to be infused. A diaphragm separates the drug from propellant, normally Freon maintained at negative pressure. The solenoid is activated driving an armature and a bellows pumping element. The displacement of the armature opens a check valve which draws drug from the storage chamber into a downstream pumping chamber. A restriction is used to prevent backflow in the outlet during this short period. When the pump chamber is full, the check valve closes and the solenoid is deenergized. A spring force is used to displace the bellows into the chamber thereby pumping the drug through a restrictor and into the patient. The bellows armature assembly comes to rest on the check valve to insure that no backflow occurs during the rest period. Such a system operates at negative pressure to insure no forward flow during this rest period, that is the drug chamber pressure is below body pressure.
Such negative pressure systems suffer from several significant disadvantages. First, the ingestion of air into the system will stop the drug flow process. Consequently, such devices require expensive fill and empty systems for recycling of the implantable device. A more practical and serious problem is that special handling is required for the devices themselves. The drugs used with such devices must be vacuum conditioned thereby requiring that special steps be taken by those who in many cases are medical technicians and are technically unsophisticated. The drugs must also be packaged and shipped with special care to maintain such vacuum conditioning. Consequently, these devices, while offering theoretical advantages, in practice suffer from significant disadvantages.
A second class of device is the so-called positive pressure pumps which are in combination with an accumulator pump. Typical are U.S. Pat. Nos. 4,221,219; 4,299,220; and 4,447,224. Such devices operate at positive pressure thereby eliminating the problems of prior art negative pressure devices. Because the drug chamber is maintained above body pressure, there is, however, a remote potential for an overdose of drug should all the valves in-line with the output fall open at the same time. An extremely high degree of safety, however, may be achieved in such systems by the use of redundant or fail-safe valves together with the addition of sensor/shut-down circuits. Such, however, results in significant cost increases which are added to the system.
Reference is made to U.S. Pat. No. 4,525,165. This patent employs a series of pump and accumulator elements between the drug chamber and the outlet. Medication is drawn out of the chamber by a pump and delivered to a bellows accumulator. This system is not only complex but cannot accurately meter doses given the variable volume of the bellows.
To deal with these situations where deficiencies exist within prior art devices, U.S. patent application Ser. No. 824,986, commonly assigned, deals specifically with a programmable positive displacement system having a pumping chamber which is placed in the path of fluid communication between the pressurized drug reservoir and a flow restrictor. By use of external programming, the device can be used to expel infusate from the pumping chamber at varying rates. While such systems provide advantages over the prior art, a need exists to define a system which is simple in operation yet provides the ability to accurately meter dosage. System simplification by the elimination of bellows chambers, implantable solenoids and the like with their attendant power requirements, represent a standing requirement for reliability and ease of operation of such implantable devices.
While not implantable devices, there exists in separate art, techniques for dispensing fluids by intravenous administration. Representative are U.S. Pat. Nos. 4,121,584; 4,261,356 and 4,262,824. These devices employ a valved accumulator metering system from a hydrostatic source using a gravity feed. The problems inherent in implantable technology are not recognized or addressed. There is similarity in the concept of using a metering chamber that is valved as in the case of the '219 patent with a diaphragm, but the application to implantable devices is not stated nor recognized in the industry.